Conventional optical projection lithography has been the standard silicon patterning technology for the past 20 years. It is an economical process due to its inherently high throughput, thereby providing a desirable low cost per part or die produced. A considerable infrastructure (including steppers, photomasks, resists, metrology, etc.) has been built up around this technology.
In this process, a photomask, or “reticle”, includes a semiconductor circuit layout pattern typically formed of opaque chrome, on a transparent glass (typically SiO2) substrate. A stepper includes a light source and optics that project light coming through the photomask to image the circuit pattern, typically with a 4× to 5× reduction factor, on a photo-resist film formed on a wafer. The term “chrome” refers to an opaque masking material that is typically but not always comprised of chrome. The transmission of the opaque material may also vary such as in the case of an attenuating phase shift mask.
The process of making the photomask begins by receiving data from a design database. The design database contains data describing at least a portion of an integrated circuit design layout, referred to as the “drawn” pattern, which generally provides a target pattern that the designers wish to achieve on the wafer. Techniques for forming design databases are well known in the art.
After receiving the design database, mask makers form one or more photomasks that can be used to implement the target pattern described by the design data. This mask making process may generally include generating mask pattern data describing initial photomask patterns for forming device features. The initial photomask patterns are formed by employing various resolution enhancement techniques. The resolution enhancement techniques can include splitting the drawn pattern so that it is patterned using two or more photomasks, such as a phase shift mask and a trim mask, for use in a phase shift process (altPSM). Methods for forming such photomask patterns from design data are well known in the art.
After the initial photomask patterns are formed, a proximity correction process is carried out that corrects the mask pattern data for proximity effects. The proximity correction process generally involves running proximity correction software to perform calculations that alter the shape of the initial photomask pattern to take into account proximity effects, such as optical diffraction effects that occur during the imaging process. In this method, a computer simulation program is often used to compute image-like model values that are taken to represent the features formed for a particular photomask feature pattern or group of patterns. Based on these simulated model values, the photomask pattern can be altered and then simulated again to determine if the altered pattern will improve the printed features. This process can be repeated until the result is within desired specifications. The features added to a photomask pattern based on this procedure are called optical proximity correction features.
After proximity correction has been performed, verification of the mask pattern data can be performed. This can include running various quality checks to determine whether the photomask patterns generated will form the desired pattern for implementing the circuit specified in the drawn data. The mask pattern data can then be sent to a mask shop, where the actual photomasks are fabricated from the mask pattern data.
One of the most common commercial implementations of alternating phase shift mask technology is the double exposure method. In this method, the critical device features to be patterned are imaged during a first exposure using a first mask, such as a phase shift mask. The non-critical and other secondary features are imaged in a second exposure using second mask, such as a conventional chrome-on-glass mask. In the past, both the first and second exposures were performed on the same photoresist layer.
More recently, a new process has been developed, referred to herein as two-print/two-etch (“2p/2e”) or “double patterning,” in which the first exposure and second exposure are each performed on separate photoresists. The patterns from each of the photoresists can be individually transferred to, for example, a hardmask. In some processes, rather than employing a hardmask, the first and second patterns from the first and second exposures can be transferred directly to the wafer in two separate etch steps.
In an exemplary 2p/2e process, a phase pattern may be formed in a first photoresist. The phase pattern can then be transferred to a hardmask using an etching technique and the first photoresist removed, A trim pattern can then be formed in a second photoresist and the resulting photoresist pattern is then transferred to the hardmask using a second etching step. Subsequently, the hardmask pattern, having both the phase and trim patterns etched therein, can be used to etch the wafer.
As device features continue to shrink, it has become more and more difficult for mask makers to implement the target patterns contained in the design database. These difficulties are generally due to spatial bandwidth constraints of modern lithography systems, and the inherent difficulties associates with forming patterns approaching a nanometer scale (e.g., such as patterns having a critical dimension of 90 nm or less). In the past, these problems have been dealt with by setting appropriate design rules that designers can follow to form a design having target patterns that can be successfully implemented. However, the design rules have become increasingly complex, and often result in complicated patterns in the target design that are difficult or impossible to implement.
Given the overly complicated patterns formed by designers, mask makers are forced to redraw the target patterns to allow them to be implemented, while still maintaining the intended functionality of the circuit design. However this can be a difficult and time consuming process due to the enormous amount of data that must be culled through by the mask makers. The overly complicated designs formed by the designers merely add to the confusion by making it difficult for the mask makers to determine the intended functionality of devices that must be redrawn. In addition, it is becoming more difficult to program the proximity correction software to successfully handle the complicated designs and produce photomask target patterns that will result in the desired target patterns set by the designers.
Accordingly, methods for more efficiently forming target patterns that can be implemented would be desirable improvements in the mask manufacturing process. Methods for improving the communication between circuit designers and mask makers regarding the intended functionality of circuit designs would also be a desirable improvement.